Microporous films or membranes have a structure that enables fluids to flow through them. The effective pore size is at least several times the mean free path of the flowing molecules, namely from several micrometers down to about 100 Angstroms. Such films are generally opaque, even when made of a transparent material, because the surfaces and the internal structure scatter visible light. When the pores are filled with a material, typically a liquid, having an index of refraction sufficiently close to that of the film material, the film can be made substantially transparent. Upon removal of the liquid, such as by evaporation, the microporous film again becomes opaque. This property of microporous films is well known in the art. Bruce N. McBane and Rostyslaw Dowbenko, Pittmentized Coatings Based on Microporous Films Obtained by Solvent Extraction, Ind. Eng. Chem. Prod. Res. Develop., Vol. 13, No. 1, at 33-37 (1974).
It has been recognized that indicia can be hidden by an opaque microporous film, and that the indicia can be rendered visible by wetting the microporous film out with a liquid to fill the micropores and rendering the film transparent. U.S. Pat. No. 4,846,095 "Critical Temperature Indicating Device" (Emslander) teaches a device for indicating a critical temperature. A microporous sheet is exposed to a mixture comprising two liquids which is incapable of wetting out the film at a given temperature, but capable of wetting the film at a critical temperature. At the critical temperature, the components of the mixture separate, allowing one of the liquids to irreversibly wet the film. Emslander does not suggest the use of the film in a security label, or the ability to provide both a temporary indication of authenticity and a permanent indication of tampering.
It has been recognized in U.S. Pat. No. 4,374,889, "Oil-Repellent Microvoid-Imaging Material" (Arens), that when a sheet material having a transparentizable opaque microvoid containing layer is transparentized with a relatively non-volatile material such as oil, a permanent mark may remain. Arens teaches a dark-colored substrate having a microvoid containing layer on both the front surface (working surface) and back surface. The front surface is treated with oleophobic fluorocarbons to prevent oil from penetrating and transparentizing the front surface. The back surface of an adjoining sheet can be contacted against the front surface to blot away oil from the from surface to prevent permanent marks. Arens does not suggest the use of microvoid containing layers in a security label adhered to an article. Furthermore, Arens teaches how to prevent permanent marks caused by oil, rather than providing a permanent indication of tampering in response to the application of oil.
It is known that a normally opaque microporous film can be rendered transparent by the application of a sufficiently high temperature to the film. The high temperature causes the microvoids to substantially collapse, reducing the internal reflection of light caused by the microvoids. It has been recognized that this property of microporous films can be used to provide an indication of tampering by the application of heat. U.S. Pat. No. 4,733,786, "Container and Innerseal Capable of Indicating Heat Tampering" (Emslander) discloses an innerseal for bottles or other containers comprising a facing layer for adhering to a container opening, an insulating layer adhered to the facing layer, and a thermally sensitive layer adhered to the insulating layer. In one embodiment, the thermally sensitive layer is normally opaque, and becomes substantially transparent upon application of heat to the innerseal. When the thermally sensitive layer is transparent, messages which have been printed on the facing layer, the insulating layer, or the side of the thermally sensitive layer adjacent the insulating layer become visible. The message is normally not visible when the thermally sensitive layer is opaque. The message provides an indication of heat tampering when the thermally sensitive layer is transparent. One preferred thermally sensitive layer taught by Emslander is the microporous sheet material disclosed in U.S. Pat. No. 4,539,256 "Microporous Sheet Material, Method of Making and Articles Made Therewith," (Shipman). However, Emslander does not suggest a security label which is temporarily transparentized with a liquid to authenticate the label, nor does Emslander suggest using a soluble tamper-indicating indicia which permanently bleeds through the thermally sensitive layer upon tampering with a solvent.
U.S. Pat. No. 5,389,426, "Article for Use in Forming a Permanent Image Using a Temporary Marker" (Arens et al.), discloses a porous material having a top surface and a bottom surface, and a dye applied to the bottom surface of the porous material. The dye is soluble in a volatile imaging liquid, such that when the volatile liquid contacts the top surface of the porous material, the volatile liquid penetrates the material, dissolves at least some of the dye to provide a dye solution, with at least some of the dye solution migrating to the top surface. When the volatile liquid evaporates, a permanently visible mark of dye residue remains on the top surface of the porous material. The dye may be of any composition, provided that the dye is soluble in the imaging liquid. Arens et al. discloses that the porous material may be of any construction provided the porosity permits the imaging liquid to penetrate the porous material from the top surface and reach the dye below. In one embodiment, Arens et al. teaches that the porous material is preferably any type of porous paper such as Mead 20 lb. coated front paper, NCR 20 lb. coated paper, or standard 50 lb. offset paper. In another embodiment, Arens et al. teaches that the transparentizable microporous layer comprises particles having an index of refraction from about 1.3 to 2.2 incorporated in a binder which has an index in the same range as the particles, with interconnected microvoids being present throughout the layer. Arens et at. also teaches that in embodiments in which the porous material is rendered transparent by the volatile liquid, there will be a visible indication that is a combination of two components. There will be a temporary component caused by rendering the porous material transparent allowing the dye to be viewed through the material, and after a sufficient amount of time, a permanent component caused by dye which has migrated to the top surface. Upon evaporation of the liquid, the porous material will no longer be transparent, and only the permanent component of the indication will remain visible. However, Arens et at. does not teach applying two indicia to the bottom surface of the material, one to provide a temporary indication which does not bleed through the material upon application of a liquid which does not cause the second indicia to bleed through, and a second which will provide a permanent indication by bleeding through the material upon application of selected solvents.
Thus, it is seen that there is no suggestion in the art to provide a tamper-indicating and authenticating label which provides a temporary visually perceptible indication that the label is authentic, and provides a permanent indication of tampering with the label, the label comprising a normally opaque microporous film which becomes substantially transparent upon application of a suitable liquid and which includes tamper-indicating and authenticating indicia which are normally not visually perceptible when the microporous film is opaque, and which become visible when the microporous film is made transparent.